This invention relates to a set accelerator for gypsum hydration. More specifically, it relates to a set accelerator that reduces the initial induction period that delays setting of gypsum.
Freshly mined gypsum is known as landplaster. It is an ore that includes calcium sulfate dihydrate, also known as gypsum, plaster or terra alba. In many deposits, landplaster includes at least 50% by weight calcium sulfate dihydrate. The calcium sulfate dihydrate is calcined at temperatures above 150° C. to drive off part of the water of hydration, forming calcium sulfate hemihydrate, also known as calcined gypsum, stucco, Plaster of Paris, calcium sulfate semihydrate or calcium sulfate half-hydrate. When added to water, calcium sulfate hemihydrate crystallizes with the water to reform the dihydrate. As the reaction proceeds, an interwoven matrix of calcium sulfate dihydrate crystals forms and becomes hard. The reaction is represented by the formula:CaSO4.½H2O+ 3/2H2O→CaSO4.2H2O
Many substances are known to accelerate the hydration rate of calcium sulfate hemihydrate hydration. Set time is determined by one of two mechanisms. There is an initial delay or induction period while a few “starter” crystals are formed. After this induction period, the rate of reaction increases to a standard rate. Most set accelerators increase the hydration rate. For example, most sulfate compounds act as set accelerators according to LeChatlier's principle. Few substances are known to reduce the induction period. The most well-known of these is calcium sulfate dihydrate.
Mined, unground landplaster is ineffective as a set accelerator for hydration reactions. Grinding calcium sulfate dihydrate exposes fresh nucleation sites that speed formation of the dihydrate gypsum. Exposure to humidity deactivates the nucleation sites in as little as a few hours. To preserve the active surfaces, it is known to treat ground calcium sulfate dihydrate with a starch, such as sugar, to prevent oxidation. When added to a slurry of calcined gypsum and water, the starch quickly dissolves and exposes the active crystal sites. The active crystal sites act as “seed” crystals that facilitate formation of calcium sulfate dihydrate crystals in an interlocking matrix. Examples of coated calcium sulfate are HRA and CSA, described in U.S. Pat. Nos. 2,078,199 and 3,573,947, respectively, both of which are herein incorporated by reference in their entirety.
Another known accelerator is described in U.S. Pat. No. 6,409,825, herein incorporated by reference. This accelerator includes ground calcium sulfate dihydrate in water with at least one additive selected from the group consisting of an organic phosphonic compound, a phosphate-containing compound or mixtures thereof. As with CSA and HRA, the gypsum particles act to accelerate the initial crystallization during the hydration reactions. This particular accelerator exhibits substantial longevity and maintains its effectiveness over time such that the wet gypsum accelerator can be made, stored, and even transported weeks or months after its manufacture. The wet gypsum accelerator is used in amounts ranging from about 5 to about 80 pounds per thousand square feet (24.3 to 390 g/m2) of board product.